CD38-negative relapse in multiple myeloma after daratumumab-based chemotherapy.

We present a case report of a patient relapsing after anti-CD38 treatment (daratumumab). The phenotype of the disease changed during this treatment, and the myeloma clone became CD38 negative and daratumumab refractory. We expected clonal shift, however, based on immunophenotyping, cytogenetics and arrayCGH; the clone was identical as before daratumumab-based treatment with the exception of CD38 negativity. We suggest that the downregulation or loss of CD38 might be an epigenetic "escape mechanism" of malignant plasma cells from antibody-based treatment. The aim of our study was to point out the pitfalls of immunophenotyping and cytogenetics in both assessing the minimal residual disease and clone detection after monoclonal antibody-based therapy.